If I see a metaphor I don't understand, I immediately think 'rhyming slang' in the first instance, because people are always making up new ones - rhyming slang is a very open-ended and rapidly changing form of our language! So I just assumed something like "Skinny dip" --> "tip". But maybe I'm Pete Tong here...?
Or as Paul Calf has been known to say it's all a "Bag O' Shite".